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LONDON, ENGLAND - NOVEMBER 20: Rafael Nadal of Spain celebrates a point during the men's singles match against David Ferrer of Spain on day six of the Barclays ATP World Tour Finals at the O2 Arena on November 20, 2015 in London, England. (Photo by Clive Brunskill/Getty Images)

LONDON, ENGLAND - NOVEMBER 20:  Rafael Nadal of Spain celebrates a point during the men's singles match against David Ferrer of Spain on day six of the Barclays ATP World Tour Finals at the O2 Arena on November 20, 2015 in London, England.  (Photo by Clive Brunskill/Getty Images)